The study here sets out to define the conditions and factors involved in the in vivo and in vitro generation of CD4 cytotoxic T lymphocytes or CD4 CTL, in addition to determining the direct roles for CD4 CTL in vivo in antiviral immunity and immune-induced pathology. Many viruses establish persistent infections, which are mostly innocuous. However, immune control of these chronic viral infections has also been linked with viral-induced autoimmunity whereas in the absence of immune protection as in immune compromised individuals, reactivation of the viruses frequently causes lethal diseases. It is not known which immune cells keep persistent viruses under control or cause immune pathology, but cytolytic CD4 effector cells have been associated with chronic viral infections as well as with viral- induced autoimmunity. However, a direct role for CD4 CTL has not been determined and their specific contributions in health and disease remain unexplored. Recently, we published a study showing that CD4 CTL form a separate effector subset distinct from any CD4 T-helper subtype. We further determined that CD4 CTL are progeny of conventional na?ve CD4 T cells that functionally reprogram in response to chronic antigen-stimulation such as chronic viral infections, and become MHC class II-restricted CTL. Using a combination of these profound new insights together with sophisticated gain- and loss-of-function approaches, we propose here to test the hypothesis that CD4 CTL play critical roles in controlling chronic viral infection and furthermore, that aberrant regulation of these cells might lead to viral-induced immune pathology and autoimmunity.